1. Field of the Invention
The present invention relates to aqueous alkaline cyanide-free baths for the galvanic deposition of zinc or zinc alloy coatings on a substrate.
2. The Relevant Technology
The deposition of zinc from cyanide, alkaline solution has dominated the industrial market for many years. The ever-increasing demands on electrodeposition plants as regards the disposal of old zinc electrolyte baths and the associated stringent controls on the effluent have led to an increased interest in the non-toxic, cyanide-free zinc electrolyte baths. Cyanide-free zinc electrolyte baths can be subdivided into two types of baths, namely weakly acidic zinc electrolytes (containing zinc chloride and/or zinc sulfate) and alkaline zincate electrolytes.
A uniformly lustrous zinc layer is deposited from weakly alkaline zinc baths, with the result that this method has rapidly captured a large part of the market. This method has the disadvantage however that its current yield is always 100% over a broad current density range.
In the case of coating pieces that have a simple shape, this may be regarded as a positive feature since the current is consumed exclusively in the deposition of zinc. However, in the case of coating parts that have a complicated shape, this leads to a thick zinc layer in the region of high current densities, and to very thin zinc layers in the region of low current densities.
The ratio of zinc layer thickness in the high current density range to the zinc layer thickness in the low current density range is termed the layer thickness distribution and in the ideal case should be 1. Zinc and zinc alloy baths always have to satisfy relatively high demands. Accordingly, a zinc layer on the object to be coated should have the same layer thickness everywhere and should exhibit a high gloss. A good layer thickness distribution can be achieved by reducing the current yield in the high current density range, while the current yield in the low current density range remains the same.
This manner of adjustment of the zinc layer thickness over a broad current density range has hitherto been successfully achieved only by the deposition of zinc from alkaline, cyanide-free electrolytes. Alkaline zinc galvanizing baths are generally based on an aqueous solution of zincate ions in sodium or potassium hydroxide. By using these baths it is possible to deposit zinc layers having a high gloss (DE 25 25 264, U.S. Pat. No. 3,884,774), although these zinc layers do not have a uniform layer thickness distribution.
Numerous proposals for improving the layer thickness distribution of the zinc layers by adding suitable additives have already been made in the prior art (U.S. Pat. Nos. 5, 405,523, 5,435,898, DE 195 09 713, U.S. Pat. No. 4,030,987).
With the hitherto proposed additives there is however the disadvantage that the galvanically produced zinc layers have a tendency to undergo exfoliation. The exfoliation of zinc and/or zinc alloy layers from the coated substrate, often also termed xe2x80x9cblisteringxe2x80x9d, constitutes a serious problem when using cyanide-free, alkaline baths, and in this connection there is still no reliable information as regards the influence of the additives used in each case on the blistering. The phenomenon of blistering is particularly disadvantageous since it often occurs only after several weeks and may therefore frequently lead to claims and litigation in the coating industry.
U.S. Pat. No. 5,405,523 describes as additive in zinc alloy baths a substance with the trade name Mirapol A 15 and similar compounds, which is said to improve the gloss of zinc alloys.
U.S. Pat. No. 5,435,898 describes as additive for zinc and zinc alloy galvanizing baths a similar compound having the trade name Mirapol WT, which likewise is said greatly to improve the layer thickness distribution.
DE 195 09 713 describes a diallyl ammonium/sulfur dioxide copolymer as additive for zinc and zinc alloy galvanizing baths, which is said to impart a uniform layer thickness to the zinc layer.
U.S. Pat. No. 4,030,987 similarly describes a diallyl ammonium/sulfur dioxide copolymer as additive for zinc and zinc alloy galvanizing baths, which is said to impart a uniform layer thickness to the zinc layer.
It has been found however that the aforedescribed additives have disadvantages in the deposition of the zinc layer, and in particular lead to blistering of the coatings.
The object of the present invention is accordingly to overcome the defects of the prior art and in particular to provide an aqueous cyanide-free alkaline bath for the galvanic deposition of zinc and zinc alloy coatings, by means of which coatings of zinc or zinc alloys can be obtained in which there is no tendency to undergo exfoliation even after prolonged storage. In this connection the advantages of these baths as regards a uniform layer thickness, a high gloss, and the uniformity of the alloy components in the coating should be retained over a broad range of current densities.